Chief, Bureau of Animal Industry.
GENERAL INTRODUCTION.
The importance, to the farmer and stock raiser, of a general knowledge of the nature of infectious diseases need not be insisted on, as it must be evident to all who have charge of farm animals. The growing facilities for intercourse between one section of a country and another, and between different countries, cause a wide distribution of the infectious diseases once restricted to a definite locality. Not only the animals themselves, but the cars, vessels, or other conveyances in which they are carried may become agents for the dissemination of disease. The growing tendency of specialization in agriculture, which leads to the maintenance of large herds of cattle, sheep, and hogs, makes infectious diseases more common and more dangerous. Fresh animals are being continually introduced which may be the carriers of disease from other herds, and when disease is once brought into a large herd the losses become very high, because it is difficult, if not impossible, to check it after it has once obtained a foothold.
These considerations make it plain that only by the most careful supervision by intelligent men who understand the nature of infectious diseases and their causes in a general way can these be kept away. We must likewise consider how incomplete our knowledge concerning many diseases is, and probably will be for some time to come. The suggestions and recommendations offered by investigators, therefore, may not always be correct, and may require frequent modification as our information grows more comprehensive and exact.
An infectious disease may be defined as any malady caused by the introduction, into the body, of minute organisms of a vegetable or animal nature which have the power of indefinite multiplication and of setting free certain peculiar poisons which are chiefly responsible for the morbid changes.
This definition might include diseases caused by certain animal parasites, such as trichinæ, for example, which multiply in the digestive tract, but whose progeny is limited to a single generation. By common consent the term "infectious" is restricted to those diseases caused by the invasion and multiplication of certain very minute unicellular organisms included under the general classes of bacteria and protozoa. Nearly all the diseases of cattle for which a definite cause has been traced are from bacteria. Among these are tuberculosis, anthrax, blackleg, and tetanus (or lockjaw). Some diseases, such as Texas fever and nagana, are traceable to protozoa, while others, like vactinomycosis and aspergillosis, are caused by fungi. Those diseases of which the cause is unknown or imperfectly worked out are pleuropneumonia, rinderpest, foot-and-mouth disease, rabies, cowpox, malignant catarrh, and dysentery.
Bacteria may be defined as very minute, unicellular organisms of a plantlike character. Their form is very simple, as may be seen from an inspection of the various species depicted on [Plate XXVIII]. The description of these figures will be found on [page 360]. The magnification there given will furnish the reader some idea of their very minute size. They multiply in two ways. The bacterium elongates and then divides in the middle to form 2 daughter cells. These go through the same process at once, and thus 4 cells are produced. The division of these leads to 8, the division of 8 to 16, and so on indefinitely. The rapidity with which this multiplication takes place depends upon the nature of the bacterium. The bacillus of tuberculosis multiplies very slowly, while that of anthrax does so with great rapidity, provided both are in the most favorable condition. Another mode of reproduction, limited to certain classes of bacteria, consists in the formation of a spore within the body of the bacterium. Spore formation usually takes place when the conditions pertaining to the growth of the bacteria become unfavorable. The spores are much more resistant to destructive agents than the bacteria which produced them. The anthrax spore may live several years in a dried state, but the anthrax bacillus perishes in a few days under like conditions. This matter will be referred to again when we come to discuss the subject of disinfection.
Of the protozoa which cause disease very little is at present known. One which produces Texas fever is pictured on [Plate XLV], in figs. 4 and 5. These parasites have a more complex life history than bacteria; and as they can not be grown in artificial media, their thorough investigation is at present hampered with great difficulties.
The differences in the symptoms and lesions of the various infectious diseases are due to differences in the respective organisms causing them. Similarly the great differences observed in the sources from which animals become infected and the manner in which infection takes place are due to differences in the life history of these minute organisms. Much discussion has taken place of late years concerning the precise meaning of the words "infection" and "contagion."